Copper beryllium alloys have been used commercially for approximately fifty years in applications requiring high strength, formability, stress relaxation resistance and conductivity. Historical development of copper beryllium alloys and the processes to manufacture them has generally proceeded in the direction of providing premium performance, i.e., the highest strengths, best ductilities and other highly desirable attributes, by taking advantage of the precipitation hardening characteristics of these alloys. Thus, U.S. Pat. Nos. 1,893,984, 1,957,214, 1,959,154, 1,974,839, 2,131,475, 2,166,794, 2,167,684, 2,172,639 and 2,289,593 disclose various wrought alloys containing varying amounts of beryllium and other elements. Commercial copper beryllium alloys include those wrought alloys bearing Copper Development Association designations C17500, C17510, C17000, C17200 and C17300.
In the fifty or so years since the above-discussed patents were granted, whole new industries have appeared and new sets of requirements have been imposed on alloy producers. Thus, the requirements of the electronics and computer industries were unknown in the 1930's. Even the trends toward miniaturization in electronics and computers have arisen and proceeded at an accelerating pace only in the past few years. In the provision of spring-type connectors and contacts, the complexity of the devices needed, and the requirements for heat dissipation, as well as for survival of parts at elevated temperatures without failure due to stress relaxation, have proceeded apace. In addition, purchasers have become increasingly price-conscious and connector alloys such as phosphor bronzes C51000 and C52100 have been employed due to cost even though the inferior performance of such alloys, such as in poorer conductivity, poorer formability and lower stress relaxation resistance as compared to copper beryllium alloys, was known. Moreover, the formability requirements imposed by the production of complex parts from strip or wire using progressive dies and other metal forming technologies, and the need for greater resistance to stress relaxation demanded by today's high reliability electrical and electronic connector, switch and relay applications, have elevated the difficulties imposed upon alloy suppliers as compared to the simpler days of U.S. Pat. Nos. 1,893,984 and 2,289,593 wherein the compositions and processing of copper beryllium alloys were intended only to obtain maximum strength-conductivity relationships, and no reference was made to considerations of formability or stress relaxation behavior.
Processes in the prior art to produce wrought forms (i.e., strip, plate, wire, rod, bar, tube, etc) of copper beryllium alloys have generally focused on the premium performance alloys with beryllium and major third element contents reminiscent of the composition of commercial alloys C17500, C17510 and C17200. These processes have generally included the steps of preparing the molten alloy, casting an ingot, converting the ingot to a wrought form by hot and/or cold working with optional intermediate anneals to maintain workability of the alloy, solution annealing the wrought form by heating to a temperature sufficient to effect recrystallization of the alloy and solid solution of the beryllium in the copper matrix and then rapidly quenching the alloy to retain the beryllium in supersaturated solid solution, optionally cold working the solution annealed wrought form a predetermined amount to enhance the subsequent age hardened strength, then age hardening the optionally cold worked wrought form at temperatures less than the solution annealing temperature to achieve desirable combinations of strength and ductility. This art is disclosed in U.S. Pat. Nos. 1,893,984, 1,959,154, 1,974,839, 1,975,113, 2,027,750, 2,527,983, 3,196,006, 3,138,493, 3,240,635, 4,179,314 and 4,425,168 which also teach that optimum solution annealing and aging temperature ranges are dependent upon alloy composition, and that age hardening may be performed either before, during or after the act of fabricating the solution annealed and optionally cold worked wrought form into an article of manufacture (e.g., an electrically conductive spring, pressure welding electrode, or similar device) by well-known metal forming technologies.
Copper-base alloys of the prior art which are not age hardenable (such as C51000 and C52100 phosphor bronzes) and which derive their strength solely from work hardening are frequently cold worked substantially beyond 50% reduction in area in order to achieve commercially significant strength levels. In the case of copper beryllium alloys of the prior art, final cold work applied between solution annealing and age hardening, other than that associated with any parts-fabrication metal forming operations, generally is confined to levels less than about 50% reduction. Thus, U.S. Pat. Nos. 3,138,493, 3,196,006, 4,179,314 and 4,425,168 describe processes involving a minimum of 3% to a maximum of 42% cold reduction prior to age hardening. One explanation of this restriction on cold work in the commercial copper beryllium alloys of the prior art is given in the 1982 publication "Wrought Beryllium Copper" by Brush Wellman Incorporated, which shows that as-rolled ductility (and hence formability--the minimum bend radius for no cracking when bent 90.degree. or 180.degree. in a forming operation) degrades to commercially unacceptable levels as pre-aging cold work increases beyond about 40% reduction, and that post-cold working age hardened strength exhibits a relative maximum at about 30% to 40% cold reduction, but decreases with larger amounts of cold work when the alloys are aged at commercially recommended times and temperatures.
Copending application Ser. No. 550,631 by Amitava Guha, assigned to Brush Wellman Inc., describes an improved process for commercial copper-beryllium-nickel alloy C17510 involving cold work of up to about 90% intermediate to a special high temperature solution annealing treatment to form a nickel-rich precipitate and a low temperature age hardening step, the whole intended to develop strength and electrical conductivity combinations previously unobtainable in C17500 and C17510, with little or no sacrifice in formability and resistance to stress relaxation. U.S. Pat. No. 2,289,593 also discloses copper-beryllium-nickel alloys cold worked in one instance as much as 80% prior to aging, but this is in reference to an alloy containing at least 1.47% Ni, and only electrical conductivity is reported.
The property of stress relaxation is an important design parameter which can give the designer assurance that a particular contact or connector or like device will maintain the required contact pressure to assure long-life performance of an assembly including the device. Stress relaxation is defined as the decrease of stress at constant strain with time for a given temperature. From a knowledge of the stress relaxation behavior of a material, a designer can determine how much the room temperature spring force must be increased to assure a particular minimum force at operating temperature to maintain electrical contact between mating parts for an extended time period.
The stronger beryllium-containing age hardenable alloys such as C17200, which contains about 2% beryllium, are known to have high resistance to stress relaxation. On the other hand, the considerably cheaper phosphor bronzes, such as C51000 and C52100, which are non-agehardenable and have to be severely cold worked to achieve high strength, are poor with respect to resistance to stress relaxation.
As used herein, stress relaxation resistance is determined by the test described in the paper entitled "Stress Relaxation of Beryllium Copper Strip in Bending" by Harkness and Lorenz presented at the 30th Annual Relay Conference, Stillwater, Okla., Apr. 27-28, 1982. In accordance with this test, flat spring specimens having a tapered gage length are stressed in a fixture to a constant initial stress level and are exposed with the fixture in the stressed condition to an elevated temperature such as 300.degree. F. (150.degree. C.) for an extended time period. Periodically, a specimen is removed and measured to determine the amount of permanent set the material has undergone, from which the percent of remaining stress value can be calculated.
Formability is determined by bending a flat strip specimen about a punch having a nose of variable known radius with failure being taken as the point at which cracking occurs in the outer fibers of the bend. A rating is given for the test from the quantity R/t wherein "R" is the radius of the smallest punch nose which causes no cracking and "t" is the thickness of the strip. The rating can be used by designers to determine whether a particular material can be formed to the geometry desired in a particular part.
The present invention provides a process to produce age hardenable copper beryllium alloys containing small amounts of nickel, where cobalt may be substituted for a portion of said nickel content, having a stress-relaxation resistance approaching that of the strongest copper beryllium alloys of commerce together with high formability and ductility, high conductivity and useful strength. Our copending application Ser. No. 623,463, "Processing of Copper Alloys", relates to a processing technique for copper beryllium alloys containing small amounts of cobalt.